Field
The present specification generally relates to the manufacture of glass articles with formed through holes and, more specifically, to methods for laser drilling through holes in substrates using an exit sacrificial cover layer, and work pieces including a substrate and an exit sacrificial cover layer.
Technical Background
Holes may be formed in substrates by methods such as laser machining, photo machining, direct molding, electrical discharge machining, and plasma/reactive etching. Such substrates may be used in a number of electrical devices, such as interposers that route electrical signals between silicon microchips and organic substrates.
In the laser drilling method, pulses of a UV laser are directed to a substrate in predetermined positions to thereby form holes in the substrate at the predetermined positions. The number of pulses applied at each predetermined position may be determined based on the desired depth of the hole in the substrate. The diameters of the holes formed by laser drilling may be modified using an optional etching process. In the etching process, an etching solution is applied to the surface of the substrate, such as by spraying or immersion, after the holes have been drilled into the substrate by the laser. The duration of exposure, temperature, concentration, and chemistry of the etching solution may be determined based upon the desired diameters of the holes in the substrate.
However, as the desired diameter of the through holes and the thickness of the substrate decreases, the exit-to-entry diameter ratio of the through holes decreases to unacceptable levels. Particularly, as the laser beam is pulsed into the glass substrate, the bottom of the hole is tapered and therefore results in a pointed portion. Accordingly, when a through hole is laser drilled by a pulsed laser beam, the exit diameter of the through hole is smaller than the entrance diameter due to the pointed portion. Even after an etching process, because the same amount of glass material is removed across all surfaces of the substrate, the exit-to-entrance diameter ratio remains undesirable due to the small diameters.
Accordingly, a need exists for alternative work pieces and methods for forming through holes having high exit-to-entrance diameter ratios in substrates.